TONOMETER

Abstract: A tonometer for measuring the intra-ocular pressure of an eye comprising an emitter (12) for emitting a signal towards the eye, a receiver (14) for receiving the signal on being reflected from the eye and a monitoring device (15) for monitoring the amplitude of the signal received by the receiver. The emitter (12) and receiver (14) are movable relative to the eye. The monitoring device (15) takes the measurement when the distance between the eye and receiver is such that the strongest signal is received. The tonometer compensates for variations in the corneal radius of the eye.

Description: TONOMETER This invention relates to a tonometer for measuring Intra-ocular pressure. Tonometry is the common procedure for measuring the Intra-ocular pressure of the eye. Many known tonometric assessments require the use of anaesthesia and contact with the eye or a short blast of high pressure air to be directed at the eye. These known methods cause a sensation in the eye resulting in discomfort to the patient. What is more contact with the eye may result in abrasion and cross infection. Repeated measurements comprising contact with the eye also have a massaging effect on the eye which may adversely affect the accuracy of subsequent measurements. The present invention has been made from a consideration of this problem. According to a first aspect of the present invention there is provided a tonometer for measuring the Intra-ocular pressure of an eye comprising an emitter for emitting a signal towards the eye, a receiver for receiving the signal on being reflected from the eye, and a monitoring device for monitoring the amplitude of the signal received by the receiver, wherein the emitter and/or receiver is movable relative to the eye while the monitoring device is operating, the monitoring device comprising means for taking and/or storing a measurement when the distance between the eye and the receiver is such that substantially the strongest signal is received, characterised in that the tonometer comprises means for compensating for variations in the cornea1 radius of the eye. According to a second aspect of the present invention there is provided a method for monitoring the Intra-ocular pressure of an eye, comprising directing a signal towards the eye, collecting the signal reflected from the eye and monitoring the amplitude of the collected signal, wherein the emitter and/or receiver is movable relative to the eye while the monitoring device is operating, the monitoring device comprising means for taking and/or storing a measurement when the distance between the eye and the receiver is such that substantially the strongest signal is received, characterised in that the method further comprises compensating for variations in the corneal radius of the eye. The complete mathematical analysis of the waveforms, angles, amplitudes etc is extraordinarily complex . However considering the situation in a very simplified way: The acoustic impedence of the incident wave is R l = p ι c ι- Where P-, = density of the air C, = velocity of sound in air The acoustic impedence of the reflected wave is R 2 = P 2 C 2 Where P^ = density of the cornea C 2 = velocity of sound in cornea It can be shown that Ar = ratio between reflected and incident amplitudes. Ar = R- a ^ - R 2 = p ι ι ~ P 2 C 2 equation 1 R 2 + R 2 P 1 C 1 + P 2 C 2 C, at normal temperatures and pressures is a constant. C 2 l- μ equation 2 d (i+ )Cι-2μ) where E = Young's modulus P = Poissons ratio d = density From this it can be seen that Ar is dependant upon Where E = Young's modulus/elasticity = stress strain where stress = force & strain = change of dimensions area original dimensions Now as Intra-ocular pressure increases the force per unit area required to deform the corneal surface increases and is therefore dependant upon E. E in turn influences C 2 and the amplitude of the reflected wave as shown in equation 1. This mathematical approach is offered by way of a simple explanation and owing to its simplified approach is not claimed to be the definitive mathematical analysis. The means for compensating for the corneal radii of the eye may comprise a mathematical equation or a table, graph or the like of known values for comparison purposes. The relevant information may be fed into a computer or the like such that the Intra-ocular pressure may be displayed instantaneously. In a preferred embodiment of the invention the tonometer comprises means for providing coherent emitted signals. The signals are preferably arranged at substantially 60° to the receiver. By arranging for coherent ultrasound sources to emit waves at about 60° to the receiver the waves interfere and superimpose creating a single wave of increased amplitude. This wave is received by the receiver which ideally takes a measurement at a distance M X" from the cornea where "X = n + / \ /2" where X=0,1,2,3 etc. At this distance the maximum amplitude of the wave occurs. The signal preferably comprises an ultrasound wave; that is a wave having a higher frequency than those detectable by the human ear. For example 40KHz waves are suitable, although other frequencies may well be appropriate. The monitoring device is preferably an oscilloscope or meter such as voltmeter. In order that the present invention may be more readily understood a specific embodiment thereof will now be described by way of example only with reference to the accompanying drawings in which:- Fig.l is a schematic illustration of one tonometer for use in accordance with the present invention; Fig.2 is a schematic illustration of the tonometer of Fig.l; Fig.3 shows the tonometer of Fig.2 in use; Fig.4 shows a longitudinal sectional view through a tonometer in accordance with the invention; Fig.5 shows a graph of corneal radius and amplitude of the wavefront directed towards the eye against the Intra¬ ocular pressure of the eye; Fig.6 shows a further tonometer in accordance with the invention; Fig.7 shows the principle of operation of the tonometer of Fig.6; Fig.8 shows a simplified analysis of the direction of the wavefronts reflected from the cornea; and Fig.9 shows a graph of peak amplitude (mV) against radius of curvature of the cornea recorded with different angles of approach of the wavefront. Referring to Figs. 1 to 4 an electric circuit produces a stable 40 KHz ultrasound wave 11 which is used to drive an emitter 12 from an oscillating high frequency electronic signal. The ultrasound waves are directed at the eye 13 from the emitter 12. The ultrasound waves are subsequently reflected from the corneal surface of the eye 13 and are then collected by a receiver 14. The receiver 14 is connected to a monitoring apparatus 15 such as an oscilloscope or after appropriate amplification and rectification the returning signal can be assessed. The amplitude of the soundwaves arriving at the receiver may be analysed by way of monitoring apparatus. By utilising a "Peak Holding" meter it is possible to "search and find" the location where the reflected signal is at its strongest and thus record and store the potential difference measured when the receiver is in that location. Prior to storage the signal is reflected and amplified in order to facilitate calibration and accurate measurement of the Intra-ocular pressure. The measured signal on the monitoring apparatus 15 is related to the rigidity of the eye and in turn to the Intra¬ ocular pressure of the eye. With reference to Fig.4 the tonometer head 20 comprises concentrically arranged inner and outer tubes 21,22. The transmitter 23 and receiver 24 are mounted in the inner tube 21. Concentric apertures 25,26 are provided at the right end of the tubes as illustrated. Here the inner receiver tube 21 extends beyond the outer transmitter tube 22. In use the transmitter 23 transmits a wave which passes between the inner and outer tubes and is reflected by the eye into the inner receiver tubes 21 and receiver 24 located therein. The measurement taken on the monitoring apparatus can be used to give the Intra-ocular pressure of the eye. The reading given by the monitoring apparatus should be compensated to allow for variations in the corneal radius of the eye. This may be taken into account in a calibration graph which can be used to derive the Intra-ocular pressure of the eye from the reading given on the monitoring apparatus. This information in the calibration graph can be fed into a computer or like apparatus so that the Intra-ocular pressure can be instantaneously displaced. The calibration graph used to derive the Intra-ocular pressure is shown in Fig.5. The graph allows for variations in the radius of curvature of the eye so as to eliminate these variations from the measured reading. The amplitude of the received wavefront is measured over a range of radii of curvature and Intra-ocular pressure. It is noted that Fig.5 is shown by way of illustration only and is not intended to represent any specific results. In use the amplitude is recorded and the corneal radius is measured. The Intra-ocular pressure can then be derived from the graph shown in Fig.5. For example, with a known corneal radius of 7.5mm and an amplitude of "X" volts the Intra-ocular pressure is observed to be 30mmHg. Alteratively the Intra-ocular pressure can be calculated mathematically, and instantaneously displayed, the mathematics taking account of the variations in the corneal radius. With reference to Fig.4 the tonometer head 20 comprises concentrically arranged inner and outer tubes 21,22. The transmitter 23 and receiver 24 are mounted in the inner tube 21. Concentric apertures 25,26 are provided at the right end of the tubes as illustrated. Here the inner receiver tube 21 extends beyond the outer transmitter tube 22. Referring to Figs. 6 to 9 a tonometer 30 comprises a transmitter 31 and a receiver 32. A wave absorbent cone 33 is provided between the transmitter 31 and receiver 32. The trans itter 31 emits two coherent wave signals "A" and "B" out of the tonometer via exits 34,35. The wave signals A,B are arranged at an angle * Q' to the longitudinal axis of the tonometer where -Λ- is 60°. The signals are directed towards the eye and are subsequently reflected from the corneal surface of the eye. They are then collected by the receiver 32. The receiver 32 is connected to a monitoring apparatus such as an oscilloscope and after appropriate amplification and rectification the returning signal can be assessed. The amplitude of the soundwaves arriving at the receiver may be analysed by way of the monitoring apparatus. By utilising a "Peak Holding" it is possible to "search and find" the location where the reflected signal is at its strongest and thus record and store the potential difference measured when the receiver is in that location. This will be when the distance "X" between the cornea and the receiver is equal to n +^/2 where ^is the wavelength of the emitted wave and n = 0,1,2,3 etc. Prior to storage the signal is reflected and amplified in order to facilitate calibration and accurate measurement of the Intra-ocular pressure. The measured signal on the monitoring apparatus 35 bears a direct relationship to the rigidity of the eye and in turn to the Intra-ocular pressure of the eye. Figs. 8 and 9 show the theory behind the operation of the tonometer of Figs. 6 and 7. Fig.8 shows a simplified geo etrical analysis of the path of the wavefronts (in 2 dimensions) considering angles "Q of 30° and 60° as reflected from a flat and a convex surface. When Q- is 30° the wavefronts are not co-incident and in particular on the convex surface separation between wavefronts B and C exists and results in a reduced amplitude when compared to a surface with a larger radius of curvature. It also can clearly be seen that when * is 60° the wavefronts A,B and C are coincident when reflection occurs from either surface, thus minimising the effect to the radius of curvature. Figure 9 illustrates the difference in peak amplitude recorded with differing values of "©" on two spherical surfaces with radii of curvature 7.00mm and 8.4mm. This indicates that as increases the influence of the radius reduces and appears to be minimised as ^ approaches 60°. It can be seen that the design of the tonometer head can be used to minimise the influence of the radius of curvature of the cornea. It is to be understood that the above described embodiment has been made by way of illustration only. Many modifications and variations are possible.

Claims: C AIMS 1. A tonometer for measuring the Intra-ocular pressure of an eye comprising an emitter for emitting a signal towards the eye, a receiver for receiving the signal on being reflected from the eye, and a monitoring device for monitoring the amplitude of the signal received by the receiver, wherein the emitter and/or receiver is movable relative to the eye while the monitoring device is operating, the monitoring device comprising means for taking and/or storing a measurement when the distance between the eye and the receiver is such that substantially the strongest signal is received, characterised in that the tonometer comprises means for compensating for variations in the corneal radius of the eye. 2. A tonometer as claimed in claim 1 comprising means for providing at least two coherent emitted signals. 3. A tonometer as claimed in claim 2 , wherein the emitted signals are arranged at substantially 60° to the receiver. 4. A tonometer as claimed in any preceding claim, wherein the signal comprises an ultrasound wave. 5. A tonometer as claimed in any preceding claim, wherein the frequency of the signal is substantially 40 KHz. 6. A method for receiving the Intra-ocular pressure of the eye using the apparatus of any of claims 1 to 4. 7. A method for monitoring the Intra-ocular pressure of an eye, comprising directing a signal towards the eye, collecting the signal reflected from the eye and monitoring the amplitude of the collected signal, wherein the emitter and/or receiver is movable relative to the eye while the monitoring device is operating, the monitoring device comprising means for taking and/or storing a measurement when the distance between the eye and the receiver is such that substantially the strongest signal is received, characterised in that the method further comprises compensating for variations in the corneal radius of the eye.

Inventors: TOLEMAN PAUL, TOLEMAN, PAUL
Applicants: TOLEMAN PAUL, TOLEMAN, PAUL
Application year: 1994
Main classification: A61B3
IPC codes: A61B3, A61B8